This application is related to Japanese patent application No.HEI9(1997)-233375, filed on Aug. 13, 1997 and a Japanese patent application filed on Jun. 11, 1998, of which application number has not been provided yet, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an electrode for a display device and method for manufacturing the same. The electrode for a display device and method for manufacturing the same of the present invention can be suitably used for electrodes of a plasma display panel (PDP), a liquid crystal display device (LCD) or the like.
2. Related Art
First, there is PDP as a typical display device in which an electrode is formed on a substrate. PDP is a self-light-emitting type display device. FIG. 7 shows a schematic slant view of PDP of a surface-discharging alternating current driving system. As shown in FIG. 7, a PDP 20 has a construction that a substrate 23 equipped with barrier ribs 21 and address electrodes A (data electrodes), each covered with a phosphor layer 22, is stuck to a substrate 27 equipped with display electrodes (each is a double-layer electrode of a transparent electrode 25 and a metal electrode 26) covered with a dielectric layer 24 made of a low-melting glass. The transparent electrode 25 is made of a transparent electrically conductive film of ITO (indium tin oxide), NESA (SnO2) or the like. The metal electrode (bus electrode) 26 has a width narrower than the transparent electrode 25 and is laminated thereon. The phosphor layers 22 are formed in a stripe form (EU in FIG. 7) and emit R (red), G (green), and B (blue) lights with the excitation of the vacuum ultraviolet light raised by gas discharge between the adjacent display electrodes. One RGB set corresponds to one pixel (EG in FIG. 7). In addition, the substrate 23 side is called a rear-side substrate and the substrate 27 side is called a display-side substrate. Also, in FIG. 7, the numeral 28 denotes a dielectric layer, 29 denotes a discharging protective layer, and D denotes a display surface.
As a method of producing the address electrode and the bus electrode, for example, a method of coating a metal paste containing Ag on a substrate by the printing method and burning to produce the electrode made of Ag and a method of producing an electrode made of three layers of Cr/Cu/Cr or Al or an Al alloy or the like, by the thin film-forming method such as the sputtering method have been known.
In the case of producing the address electrode and the bus electrode by utilizing the printing method, there was a problem that the formation of high-precision patterns having a width from about 10 to 20 xcexcm is difficult. Also, in the case of using a thin film-forming method used for manufacturing semiconductor devices, it was possible to form high-precision patterns, but there was a problem that the production apparatus, materials, or the like, are more expensive than those of other methods. Furthermore, because Cu has a property that it is liable to be diffused in a low-melting glass, there was a possibility that in the electrode made of three layers of Cr/Cu/Cr, Cu exposed at the side surface is diffused in the formation of the dielectric layer at a temperature ranging about the softening point of the low-melting glass. By diffusing Cu, there was a problem that the low-melting glass is colored and color purity of color display is deteriorated.
As a method of solving these problems, the method described in Japanese Unexamined Patent Publication (Kokai) No. 8-227656 is known. Practically, the electrode is produced by the method shown in FIGS. 11(a) to 11(d).
In the method shown in FIGS. 11(a) to 11(|d), first, a Ni layer 31 is formed on a substrate 30 [see, FIG. 11(a)]. Then, a resist layer 32 is coated on the whole surface of the Ni layer 31, and an opening is formed on a desired region of the Ni layer 31. Thereafter, a Cu layer 33 is formed in the opening by the electroplating method [see, FIG. 11(b)]. Then, the resist layer 32 is removed, and after patterning the Ni layer 31 in a desired form [see, FIG. 11(c)], a Ni layer 34 is selectively formed on the surface of the Cu layer 33 by the electroless plating method, whereby the bus electrode made of the Ni layer 31, the Cu layer 33 and the Ni layer 34 can be formed [see, FIG. 11(d)].
Also, as another method, the method described in Japanese Unexamined Patent Publication (Kokai) No. 8-222128 is known. Practically, the electrode is produced by the method shown in FIGS. 13(a) to 13(c). In this method, a transparent electrode 42 is formed on a substrate 41 in a desired form, then a Ni layer 43 is formed on the whole surface of the substrate 41, and further, a Cu layer 44 is formed on the Ni layer 43 in a desired form [see, FIG. 13(a)]. Thereafter, the Ni layer 43 is etched so that the Ni layer has the same plane form as the Cu layer 44 [see, FIG. 13(b)], and a resist layer 45 is formed and opened so that the Ni layer 43 and the Cu layer 44 are exposed. Thereafter, a Ni layer 46 is formed by the plating method so that the Ni layer 46 covers the Ni layer 43 and the Cu layer 44.
The methods described in the former publications have an advantage that an electrode of a high-precision pattern can be easily produced at a low cost.
However, in the case of covering the electrode with a dielectric layer made of a low-melting glass by sintering a low-melting glass paste, because the electrode is heated to a high temperature at sintering, there was a problem that Cu and Ni of the electrode material are mutually diffused to form an alloy, thereby increasing the resistance. The point that Cu and Ni form an alloy is shown in the phase diagram of Cuxe2x80x94Ni in xe2x80x9cConstitution of Binary Alloysxe2x80x9d, (Max Hansen, 2nd Ed., page 602, published by McGraw-Hill Book Company). This literature shows that, because the completely mixed state of Cu and Ni is thermodynamically stable, they can be easily mixed to form an alloy thereof upon heating at sintering.
Now, FIG. 12(a) is an SEM (scanning electron microscope) photograph of the cross-section of an electrode made of Ni/Cu/Cr from the substrate side, and FIG. 12(b) is an SEM photograph of the cross-section after heating the above-described Ni/Cu/Cr at 600xc2x0 C. for 40 minutes. FIG. 12(b) shows that Cu and Ni are diffused to form an alloy thereof.
However, in the method in the later publication, there was a further problem that, because the number of steps is increased, the production cost is increased.
An object of the present invention is to provide an electrode whose resistance does not increase by forming an alloy or the like and to produce the electrode without increasing the number of steps in the production. Further, in the case of covering an electrode with a dielectric layer, an object of the present invention is to provide such an electrode that coloring of the dielectric layer can be prevented and a method for manufacturing the same, the coloring caused by diffusion of metals constituting the electrode.
That is, according to a first aspect of the present invention, there is provided an electrode for a display device, comprising a laminate of an underlying layer, a conductive layer and a protective layer formed on a substrate in this order from the substrate side in such a manner that at least the conductive layer is completely covered by the protective layer, the underlying layer and the protective layer being composed of a metal which is hard to form an alloy or an intermetallic compound with the metal constituting the conductive layer and has a low solid solubility to the conductive layer or an alloy thereof.
Also, according to a second aspect of the present invention, there is provided an electrode for a display device for a plasma display panel, comprising a transparent electrode and a bus electrode which has a narrower width than the transparent electrode and is completely covered by the transparent electrode, the bus electrode being a laminate of an underlying layer, a conductive layer and a protective layer formed on a substrate in this order from the substrate side in such a manner that at least the conductive layer is completely covered by the protective layer, the electrode being covered by a dielectric layer made of a low-melting glass, the underlying layer and the protective layer being composed of a metal which is hard to form an alloy or an intermetallic compound with the metal constituting the conductive layer and has a low solid solubility to the conductive layer.
Furthermore, according to a third aspect of the present invention, there is provided a method for manufacturing an electrode for a display device, comprising the steps of: forming an underlying layer on a substrate;
forming a conductive layer on the underlying layer;
forming a protective layer on the conductive layer by an electroless plating method in such a manner that the conductive layer is completely covered by the protective layer, wherein the ionization tendency of the metals constituting the underlying layer, the conductive layer and the protective layer becomes larger in the order of the underlying layer, the protective layer and the conductive layer.
Further, according to a fourth aspect of the present invention, there is provided a method for manufacturing an electrode for a display device, comprising the steps of:
forming an underlying layer on a substrate;
forming a conductive layer on the underlying layer;
forming a protective layer on the conductive layer by an electroless plating method in such a manner that the conductive layer is completely covered by the protective layer, wherein the ionization tendency of the metals constituting the underlying layer, the conductive layer and the protective layer becomes larger in the order of the underlying layer, the protective layer and the conductive layer, thereby forming a bus electrode comprised of the underlying layer, the-conductive layer and the protective layer; and
forming a transparent electrode on the bus electrode in such a manner that the bus electrode has a narrower width than the transparent electrode and is completely covered by the transparent electrode.